1. Field of the Invention
The invention relates to the field of projectiles with trajectory correction by lateral gas jets.
2. Description of the Prior Art
Devices to achieve corrections of this kind are already known in the prior art. They may be classified under two categories. The first category comprises devices for which the axis of the lateral gas jet passes through the center of gravity of the projectile. These trajectory correctors do not, in principle, induce any moment of force in the projectile. They enable the function of attitude control under force. The modification of trajectory results from the composition of the axial velocity of the projectile and the lateral velocity resulting from the guiding gas jet. The only action affecting the orientation of the axis of the projectile arises when the projectile takes an angle of incidence following the variation of the velocity vector. The axis of the projectile gets reoriented in parallel to the velocity vector after a period of oscillations that depends on the aerodynamic stability of the projectile and the duration of the guiding impulse.
The second category of trajectory correctors using gas jets comprises projectiles in which the gas jets convey a moment of rotation to the projectile. The greater the lever arm of the gas jets, which can be likened to the distance between the point of application of the lateral jet and the center of gravity of the projectile, the greater is this rotation. In order to make the rotation stop at the desired position, there is provided a second gas jet exerting a moment opposite to the first one. The values of the lever arm, the total impulse and the instance of firing of each impeller, which are set on the basis of the characteristics of the projectile, enable, in principle, the following factors to be controlled all at once:
the cancellation of the angular velocity of the projectile; PA1 the deviation of the velocity; PA1 the angular position (yaw and pitch) of the projectile. PA1 It1 L1=It2 L2 PA1 It1, It2: total impulse delivered by each impeller; PA1 L1, L2: lever arm of each impeller. PA1 It1.about.F1 t1 PA1 It2.about.F2 t2 PA1 giving It, the sum of the total impulse values. We have: PA1 It1=It (L1/(L1+L2)) PA1 It2=It (L2/(L1+L2)) PA1 either by increasing the thrust; PA1 or by splitting up the thrust among several corrections.
For the guidance, therefore, a juxtaposition of pairs of impellers is made, their number being given by the maximum number of guidance corrections envisaged.
The impellers may be laid out longitudinally as shown in FIG. 1. This figure gives a schematic view of a longitudinal section of a projectile. The impellers are laid out along the longitudinal axis by pairs of impellers a, a', b, b', and c, c'.
The impellers of each pair are laid out on either side of the center of gravity G of the projectile.
The condition of cancellation of the angular velocity of the projectile dictates the following relationship between the parameters of the same pair of impellers;
with
It may be recalled that the total impulse is the integral in time of the force delivered by the impeller during its operation. Should the forces F1 and F2 of the two impellers be substantially constant, the following are obtained:
where t1, t2 are the combustion times of the impellers.
Thus, for an equal lever arm, the impellers of the same pair may be identical. Most generally, the total impulse of each impeller is inversely proportional to the lever arm.
The deviation of the velocity for a guidance correction imposes a value on the sum of the total impulse values for each pair of impellers;
It=It1+It2
The value of the total impulse of each impeller (It1, It2) is deduced therefrom as a function of the lever arms (L1, L2), and of It:
The angular position, yaw and roll, after correction, depends on the preceding parameters (It, L1, L2), and the firing sequence of the two impellers used for this correction.
It is generally laid down that the two impellers should never be in operation at the same time, in order to prevent excess lateral load factor and couplings (interaction between the jets) between the effects of the two impellers.
The limit therefore is that the firing of the second impeller must follow the extinction of the first one.
In this case, there is a maximum value of the lever arm, depending on the angle (yaw and roll) that must be taken by the projectile, on the inertia and on the total impulse It.
This maximum value may be reached for impellers that are at a distance from the center of gravity G. This fact makes this approach impossible or makes it necessary, for example, to diminish the amplitude of the guidance corrections asked for, to the detriment of the performance values.
For this reason, the impellers that are at a distance from the center of gravity G are shown in FIG. 1 as being smaller than the near impellers.
In other known embodiments, impellers exerting a moment of rotation on the projectile are laid out in a radial position.
This mode of layout is shown schematically in FIGS. 2a, 2b and 3a, 3b. Each of these figures shows a longitudinal sectional view of a projectile section (FIGS. 2b and 3b) and a cross-section made on a round element d, comprising impellers positioned radially (FIGS. 2a and 3a).
The round elements d, d' are positioned on either side of the center of gravity G of the projectile. The lever arm of each of the impellers are then identical and the impellers e, e' may be identical.
This type of layout has several drawbacks. The volume available for each of the impellers is limited to the portion of angular sector devoted in each of the round elements d, d' to each of the impellers, for example PI/3 for a round element having six impellers as shown in FIG. 2a or 3a. It may be sought to compensate for this constraint by increasing the length of the propulsive charge of each of the impellers. However, there soon arise constraints dictated by the section of the gas passage which must be sufficient throughout the length of the charge in order to prevent erosive combustion.
It may be noted that this minimum section increases from the charge side opposite the nozzle up to the charge side near to the nozzle hence in the direction of the discharge of the combustion gases. The filling rate (the volume of the charge with respect to the volume of the combustion chamber of the impeller) is then penalized.
Furthermore, whatever the shape of the impellers, the volume of the round element cannot be used in an optimum manner. This penalizes the mass balance. If the section of the combustion chamber is circular as shown in FIG. 2a, the penalizing of the mass balance results from of the unused volumes between the impellers. If the shape of the section of the impellers is petal-shaped as shown in FIG. 3a, it is more difficult to make precisely because of the shape, and there are concentrations of stresses on the walls which must be supported by the local addition of matter, which also penalizes the mass balance.
In the face of these prior art approaches, the present invention is aimed at a projectile guided by means of gas jets having both the advantages of the round element arrangement as shown in FIGS. 2a, 2b or 3a, 3b and those of the longitudinal arrangement as shown in FIG. 1 without having the drawbacks of either arrangement.
Advantageously, the guided projectile according to the invention has only two nozzles, one on each side of the center of gravity and a plurality of impellers distributed in pairs. For each pair of impellers, one impeller emits its gases by one of the nozzles and the other by the second nozzle.
Since there are only two nozzles the lever arms are identical for each of the corrections.
The reduction of the number of nozzles is an obvious advantage with respect to their integration into the projectile.
First of all, the entire available section of the projectile (generally the circular section except for the central core) can be used to make the impeller. The cylindrical shapes that result therefrom are simple shapes that can be easily made and provide for high mechanical strength (resistance to pressure).
Furthermore, while the sum of the volume of the chambers of the embodiment according to the invention remains close to the sum of the volumes of the chambers of several impellers, a major gain is obtained from the number of nozzles. This number can be reduced to two. It may even be advantageous to increase their size to increase the thrust delivered and thus improve the performance characteristics (or reduce the pyrotechnic charge mass).
The identical lever arm of each of the sets prevents the restrictions laid down on the impellers located far from the center of gravity in embodiments of the type with "longitudinal layout of impellers".
It may be recalled that, to the value of total impulse dictated by the guidance performance characteristics (action on velocity), there corresponds a maximum value of the lever arm. In the proposed approach, the only constraint laid down as regards the mechanical design of the projectile is therefore the longitudinal position of two nozzles. It would appear that this constraint can be easily complied with.
Furthermore, in applications to projectiles driven by a rolling velocity, the proposed approach is always quite appropriate.
Indeed, if the projectile has a rolling velocity, the duration of operation of the impellers must be small so as not to "average" the correction on an excessively great roll angle (a correction on one rotation is wholly ineffective). In this case, the total impulse needed for a guidance must be obtained:
The increase of the thrust (with total impulse maintained) has little affect on the quantity of pyrotechnic charge but above all modifies its surface area and its combustion speed. However, it calls for an increase in the gas flow rates, hence an increase in the size of the nozzles. The proposed approach comprising only two nozzles is consequently quite appropriate.
The splitting up of the thrust is also easier with the proposed approach than with prior art approaches: the splitting up relates only to two impellers as compared with twice the number of guidance corrections in the other approaches.